1. Field of the Invention
This invention relates to a measuring device for measuring relative movement between two objects, and more particularly to such a measuring device which is adapted to accomplish an improvement in resolution.
2. Description of the Prior Art
In a machine tool or the like, accurate measuring of relative movement of a tool to a work which is an object to be worked is highly important to the precision working of the work. Likewise, in an industrial recorder or the like, accurate measuring of relative movement between a record medium and a record head is required to accurately control movement of the record head to the record medium depending upon an input signal.
In order to meet such a demand, various systems for measuring movement between two objects relatively moved to each other have been proposed and put to practice.
One of such systems proposed is a measuring device which utilizes a moire fringe formed by two optical lattices superposed. More particularly, the measuring device, as shown in FIG. 7, is so constructed that a main scale 101 including an optical lattice 102 comprising transmission portions and nontransmission portions alternately arranged at a predetermined pitch and an index scale 103 formed with an optical lattice 104 of the same pitch which are positioned opposite to each other in such a manner that a microinterval is defined therebetween and both optical lattices are crossed to each other at a microangle. Also, the measuring device includes a light source 105 arranged on one side thereof and a photocell 106 arranged on the other side thereof in a manner to interpose both scales 101 and 103 therebetween. The irradiation of light from the light source 105 to both scales 101 and 103 forms a moire fringe, which is then detected by the photocell 106.
In the measuring device described above, relative movement between both scales 101 and 102 causes the moire fringe itself to be moved as well. Also, the direction of movement of the moire fringe is varied depending upon the direction of relative movement between both scales 101 and 102. The device utilizes such phenomena as described to read the moire fringe at positions different in phase by 90 degrees from each other to count the direction and amount of movement of the moire fringe, to thereby detect relative movement between both scales 101 and 103. Two signals obtained due to the reading are referred herein to as "A phase signal" and "B phase signal", respectively. More particularly, as shown in FIG. 8, the A phase signal and B phase signal obtained from the photocell 106 are supplied to amplifiers 107A and 107B, and supplied to wave shaping circuits 108A and 108B for waveform shaping, respectively. Then, the signals are introduced into a direction discriminating circuit 109, which then detects the direction of movement of the moire fringe and alternatively generates an upcount pulse UP when the moire is moved in a right direction and a downcount pulse DOWN when it is in a left direction. The so-generated pulse UP or DOWN is counted in a counter 110.
Further, other conventional measuring systems include a system using a magnetic lattice, an electromagnetic induction type system and the like.
An improvement in resolution in the above-described measuring device using the optical scales is generally accomplished by narrowing the pitch of each of the optical lattices 102 and 104.
However, the reduction of the pitch has its limit. For example, even photoetching techniques limit the reduction to as small as several microns.
In view of the foregoing, an increase in resolution of such a measuring device has been performed according to a so-called interpolation method which carries out addition or subtraction between the A phase signal and the B phase signal to produce a multi-phase signal. The interpolation method divides the pitch of the moire fringe into 8 or 16 parts to improve resolution.
However, basically the interpolation method divides a voltage level of each of the A and B phase signals at each phase, so that the number of divisions is subject to restriction. More specifically, it is substantially impossible to divide a level of each of the A and B phase signals having a crest value of about 1-2V into, for example, 100 parts for discrimination.
Recently, it has been desired to provide various kinds of industrial machines with a satisfied measuring capability so that they may exhibit higher accuracy. For example, it has been demanded that a measuring device to be installed in such machines has a capability of exhibiting resolution of as small as 0.1 .mu.m or generating one upcount or downcount pulse when relative movement between both scales 101 and 103 is carried out by a distance of 0.1 .mu.m. Unfortunately, a method of reducing or narrowing a pitch of an optical lattice and an electrical interpolation method as described above fail to provide a scale of a level sufficiently practical to meet such requirements.